there return of a read operation that times out with no data received
is inconsistent:
```
Adafruit CircuitPython 7.3.0-beta.1-31-g73f6b4867-dirty on 2022-04-30; Adafruit Feather RP2040 with rp2040
>>>
>>> import board, busio
>>> print(board.UART().read(5))
None
Adafruit CircuitPython 6.3.0 on 2021-06-01; FeatherS2 with ESP32S2
>>> import board,busio
>>> print(board.UART().read(5))
None
Adafruit CircuitPython 7.3.0-beta.1 on 2022-04-07; Adafruit Feather STM32F405 Express with STM32F405RG
>>> import board, busio
>>> print(board.UART().read(5))
None
Adafruit CircuitPython 7.3.0-beta.1-31-g73f6b4867-dirty on 2022-04-28; Teensy 4.1 with IMXRT1062DVJ6A
>>> import board, busio
>>> print(board.UART().read(5))
b''
```
Since I have a PR on this file anyway, I thought I would put in the change to make it consistent
with the other 3 board types I tried. Can not say about any of the others.
.. and enable it on atmel-samd and raspberrypi. On trinket_m0 this saves
96 net bytes of flash. There are 216 bytes actually saved by reducing
the flash storage size of the property descriptors, but added code in
several paths takes back over half of the 'raw savings'.
By organizing the "get-only" and "get-set" (but no delete) properties
each in a different section, we can represent then more efficiently.
Testing performed: that a get-only property can still be gotten but
can't be set or deleted; that a get-set property can sill be gotten or
set but can't be deleted. Tested on pygamer.
Because this requires linker file support, I only enabled it on two of
the ports.
As I mentioned in issue #6310 while investigating that the Teensy port
did not support RS485_dir pin on normal GPIO pins, I found that it
was not implemented either as well on some other ports.
So was curious to implement it for RP2040 using same approach as I did
for the MIMXRT in the Pull Request #6328
That is I setup the specified pin as a normal GPIO pin in output mode
and then when you do a write operation it sets the GPIO pin logically
high, and when the write completes I set it logically low.
Note: knowing when I can set it low can be tricky, as you need to make
sure the full output has completed otherwise the data will be corrupted.
I am using: uart_tx_wait_blocking(self->uart);
Which looks like it is supposed to wait until the busy status is no
longer set, which the Reference manual mentioned, but this is leaving
the line logically set longer than I would like.
however I have tried running it with my hacked up version of the
Python Robotis DynamixelSDK and was able to talk to some AX servos.
I did have to change the library slightly for the RP2040, as the
library was erroring out when you did something like uart.read(5)
and it timed out without receiving anything. The RP2040 returned
None whereas I think the Teensy returned an empty set, which is what
it looks like the PySerial original code expects.
Not sure if anyone is interested in this, but thought i would
put it out as PR and see.
